Kodak and others demonstrated the production of zinc sulfide (ZnS) windows years ago by hot pressing ceramic powders. This method was subsequently replaced by a chemical vapor deposition (CVD) process in military applications where cost was not an important consideration. The CVD process is advantageous in that it allows ZnS windows to be fabricated in larger, flat or curved geometries which readily conform to the shape of aircraft.
Optical grade ZnS may be deposited or grown in a fully dense state by the CVD process. The resulting ZnS is polycrystalline and orange in color. Although this CVD-processed ZnS is transparent over a portion (about 0.5 to 12 microns) of the infrared region, it has poor transmittance in the visible region of the spectrum, due to scattering effects. This shortcoming may be addressed by annealing the ZnS. Thus, the transparency of the CVD-processed ZnS may be improved, in both the visible and infra-red regions of the electromagnetic spectrum, by subjecting the material to a hot, isostatic press (HIP). The ZnS formed by this process is sometimes referred to as “multispectral” ZnS.